1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to a multi-domain LCD device that improves color characteristics and gray inversion regardless of a viewing angle.
2. Discussion of the Related Art
In general, an LCD device includes lower and upper substrates. In the lower substrate, a plurality of pixel regions are arranged in a matrix shape, and each pixel region has a thin film transistor (TFT) and a pixel electrode. In the upper substrate, a common electrode and a color filter layer for representing colors are formed thereon. A liquid crystal is injected between the lower and upper substrates. A polarizer is respectively formed at both sides of the lower and upper substrates to linearly polarize visible rays.
Such LCD device is widely used in monitors for computers, audiocassette recorders, and videocassette recorders. The LCD device contains liquid crystal therein, so that a main viewing angle is determined by a rubbing direction. An amount of light that passes through the LCD device is controlled by using birefringence and linear polarization of the liquid crystal, thereby displaying images.
In order to yield the LCD device having uniform luminance and high contrast ratio, the liquid crystal is aligned between the substrates in one direction.
There are various kinds of alignments in the liquid crystal, such as a homogeneous alignment, a homeotropic alignment, a tilted alignment, a twisted alignment, a hybrid alignment, a grandjean alignment, and a focal-conic alignment.
For the purpose of simplicity, only the twisted alignment LCD device will be described herein. In the twisted alignment LCD device, liquid crystal molecules are arranged in parallel to the lower and upper substrates. However, an alignment direction is different by an angle of 90 degrees between the lower and upper substrates, so that the molecules are successively aligned at the angle of 90 degrees between the two substrates.
Operation of such LCD device is based on an electro-optical method, in which a voltage is applied to a liquid crystal cell to vary the alignment of the liquid crystal, thereby varying electro-optical characteristics of the liquid crystal cell in representing images.
A viewing angle is an angle of eyes of a viewer with respect to the surface of the LCD device. Since a path of light passing through the liquid crystal layer varies with the angle of eyes of a viewer, birefringence of the liquid crystal is generated, thereby varying transmittance.
A viewing angle characteristic is a degree of the transmittance (contrast ratio) of the LCD device varying with the viewing angle.
Meanwhile, a related art lateral field induced vertical aligned (LFIVA) mode is a main stream technology in TFT-LCD devices. The LFIVA mode can achieve a wide viewing angle of 140 degrees or greater in any directions.
In such a mode, an alignment film on a common electrode is rubbed, and a pixel structure is formed in such a manner that a transparent electrode is etched in a portion of a pixel electrode at a width of 10 μm to 15 μm to form a slit pattern. Thus, electric fields are generated vertically and laterally. To the end, a wide viewing angle is obtained, so that a transmittance of 80%, a response time of 20 msec, and a viewing angle of 140 degrees or greater are achieved in the LFIVA mode.
In the LFIVA mode, the pixel electrode is formed of a metal film unlike the other mode using a transparent conductive film for the pixel electrode. Thus, one process step for forming a TFT-LCD can be reduced in the LFIVA mode. Especially, a scanning line can be formed of a material the same as that of the pixel electrode. Therefore, a large sized LCD TV is expected to simplify structures and reduce residual images.
A related art LCD device will be described with reference to the accompanying drawings.
FIG. 1 is a cross-sectional view showing a structure of the related art LFIVA mode LCD device.
As shown in FIG. 1, in the related art LCD device, a first transparent electrode 12 (pixel electrode) is formed to have slit patterns 11 at a constant distance on a first substrate (not shown). On a second substrate (not shown) opposing the first substrate, a second transparent electrode 13 is formed. Liquid crystals 14 are injected between the first and second substrates.
In the LFIVA mode LCD device, the liquid crystals 14 are arranged to oppose each other with respect to the slit patterns 11.
FIG. 2 is a graph showing a relationship between a gray level and a viewing angle in the related art LFIVA mode LCD device. As shown in FIG. 2, in the related art LFIVA mode LCD device, a transmittance curve of the middle gray level is significantly changed with an increase in the viewing angle.
As described above, the related art LFIVA mode LCD device has the following problems.
The liquid crystal layer in the middle portions of the slit patterns and the electrode is rotated along the rubbing direction, so that a multi-domain is formed. At this time, in the middle portions of the slit pattern and the electrode, optical distortions are generated, so that the viewing angle is inclined. For this reason, color characteristic and gray inversion are deteriorated.